Numerous industrial processes utilize combustion units, such as, for example, furnaces, ovens, incinerators, driers, boilers, flares, heated baths, and the like. The combustion units typically have multiple bumers. Each of the burners produces a flame from combustion of a fuel, such as, for example, gas, oil, coal, coke, or the like, with air, oxygen, oxygen-enriched air, or the like. The burner flames can be ignited by an associated pilot flame. In the event of a flame failure, the fuel and air supplied to the burner must be stopped to avoid a buildup of fuel in the unit, which might otherwise result in possible uncontrolled combustion or explosion. To accomplish the burner shut down, the combustion unit generally has a control system associated with each bumer, which uses a flame sensing apparatus and flame sensor circuitry for sensing the presence of the flame. Upon detecting the absence of a flame within the unit the flame sensor transmits a signal through the sensor circuitry to the burner control system that shuts the burner down. When one of the burners has been shut down, but more commonly after several of the burners have been shut down, depending on the operating characteristics of the combustion unit, the entire combustion unit is shut down primarily to prevent a hazardous situation, as well as for maintenance and repair of the improperly functioning bumers.
To present, the principal types of flame sensing transducers have been devices using the photoelectric effect, photosensitive conductors, and flame rods. Devices using the photoelectric effect generate an electrical voltage when a material is exposed to light. Photoelectric detectors are not restricted to sensing visible light, as they can be made to respond to infrared and ultraviolet radiation. Depending upon the amount of illumination detected, the photoelectric detectors send a voltage (or current in an attached circuit) of corresponding magnitude. Devices using this effect are known as photocells, photovoltaic cells, photosensors, or light-sensitive detectors. Examples of such devices are found in U.S. Pat. No. 5,245,196 to Cabaffin, U.S. Pat. No. 4,904,986 to Pinkaers, U.S. Pat. No. 4,591,725 to Bryant, U.S. Pat. No. 4,395,638 to Cade, U.S. Pat. No. 3,820,097 to Larson, and U.S. Pat. No. 3,742,474 to Muller.
Devices using the photoelectric effect have several disadvantages. The sensors are adversely affected by any dust accumulating thereon and must be periodically purged with air to remove the dust. Further, the sensors must be positioned in close proximity to the burner, and as a consequence are generally required to withstand the high-temperature environment and be explosion proof. Additionally, the sensors must be switched out if the fuel supply is changed. For example, if switching fuels from gas to oil, the sensors must usually be changed to detect the different corresponding visible, ultraviolet, or infrared spectra associated with the particular fuel.
Photosensitive conductors use compounds such as cadmium sulfide and cadmium selenide that are electrically sensitive to the flame. Such compounds decrease in resistance when exposed to light. One example of a device using photosensitive conductors is U.S. Pat. No. 2,911,540 to Powers. The photosensitive conductor is connected through a high resistance to a direct voltage source in such a manner that when and as the intensity of the radiation increases, the voltage across the conductor decreases. The voltage drop across the conductor is measured and a system shut down is performed depending upon predetermined voltage levels.
Flame rods use the flame conductivity as a detection means. The flame rod is placed in direct contact with the flame produced on a burner and a voltage is applied between the flame rod and the bumer. A current results between the flame rod and the burner due to the presence of charged particles in the flame. The current is dependent upon conditions of combustion such as input rate and air-to-fuel ratio. Measuring the current levels with the flame rods enables the detection of flame malfunction. Examples of flame rods are found in U.S. Pat. No. 5,300,836 to Cha, the 1996 Honeywell Flame Safeguard Catalog and sales literature from Gay Engineering & Sales Company, Inc.
Systems using flame rods have inherent disadvantages associated with the thermal degradation of the rod itself. Because the tip of the flame rod remains in constant contact with the flame produced by the burner or pilot flame, the flame rod is subjected to constant thermal degradation. Depending upon the type of flame rod used, the combustion characteristics and flame temperature, the life span of the flame rod may be as little as 6 months. Thus, a change in current level associated with the flame rod circuitry may be the result of the degradation of the flame rod itself, rather than any change in the flame. Additionally, because the flame rods have a relatively short life span, the combustion unit must either be periodically shut down to replace the flame rods or the operator must wear a thermally protective suit to change out the flame rod while the furnace remains on line.
To overcome the above disadvantages of relying upon a single sensor such as a flame rod, many systems use more than one type of flame sensing transducer. For example, flame rods are used to monitor pilot flames, while ultraviolet and/or infrared transducers are used to simultaneously monitor main burners. Systems incorporating more than one type of flame sensing transducer are shown in U.S. Pat. No. 5,952,930 to Umeda et al., and U.S. Pat. Nos. 5,549,469 and 5,548,277, both to Wild.
All of the abovementioned flame sensing transducers, and systems of the same, have the common disadvantage of requiring multiple sensing devices for multiple burners. In other words, each flame monitoring device is capable of monitoring only a single bumer. If, for example, there are eight burners within the combustion unit, then there must also be eight flame sensing transducers.
There exists, therefore, a need for a flame detection apparatus and method which can utilize a single flame sensing apparatus to detect failure of multiple burners.